1. Field of the Invention
An aspect of the present invention relates to a belt unit and an image forming apparatus using the same.
2. Description of the Related Art
An image forming apparatus is generally classified into a tandem image forming apparatus and an image forming apparatus of four-rotation type. In the tandem image forming apparatus, an intermediate transfer element or a transfer belt for sheet conveyance purpose is used. The transfer belt usually rotates upon receipt of rotational force from at least one drive roller supporting a belt. Meanwhile, in the four-rotation type image forming apparatus, a transfer belt to which tensile force is applied and in which a drive roller is equipped is used as an intermediate transfer element.
When one of the rollers equipped in the transfer belt are twisted with respect to the other one during the transferring operation, the transfer belt moves in a direction orthogonal to the rotational direction; that is, so-called deviation to one side occurs. A method for bringing a flange into contact with an end face of the transfer belt, to thus regulate deviation of the belt, is generally used frequently to prevent deviation of the transfer belt.
As another method for preventing deviation of a belt, a steering roller is used as one of rollers among which a transfer belt is suspended (see; for example, JP-2005-292480-A and JP-2006-65056-A). According to the method, the steering roller is provided, and hence, when the transfer belt deviates to one side, one roller shaft is tilted by a motor, thereby making a correction to movement of the transfer roller in an opposite direction.
All of the above-described related-art methods present a problem in terms of the durability of the belt and the cost of components. According to the method that brings the flange into contact with the end face of the belt, the end of the belt keeps performing movement while contacting the flange, so that the end of the belt will wear out and encounter a decrease in durability. When force for moving the belt in one direction becomes greater, abrasion or cracking (rupture) takes place in the end of the transfer belt, which in turn induces variations in rotation of the transfer belt or sometimes cause defects in an image. Therefore, to reduce the deviation force on the belt, it is generally required to manage, with high accuracy, a difference between right and left belts in terms of a peripheral length, the thickness of the belts, and alignment among rollers. The necessary entails an increase in the price of components and presents an obstacle in cost reduction.
In the meantime, according to the method providing the steering roller, a sensor for detecting the position of the end of the belt, a motor for tilting the roller shaft, a controller for controlling the motor in accordance with information detected by the sensor, and the like, are required. Therefore, the number of components is increased, which in turn entails an increase in the size and cost of the apparatus.